Surface plasmon sensors are a class of sensors with a long and commercially successful history. They are principally used to detect small amounts of biological entities. The active element of a surface plasmon sensor is a metal film. In a typical surface plasmon sensor the metal surface is prepared with an antibody to a particular protein bound to the metal surface. The angle of surface plasmon coupling is determined and then the film is exposed (usually by the use of a flow cell) to a sample that is being tested for the targeted protein. If the protein is present in the sample solution it binds to the antibody adding a dielectric-loading layer to the metal surface. This extra layer leads to an alteration in the angle of surface plasmon coupling, thus indicating the presence of the protein. The metal film thickness is typically about one tenth of the wavelength of the incident light. At angles of incidence, θ, greater than the angle for total-internal reflection the light creates an evanescent field that can penetrate through the metal. Surface plasmons are resonantly generated at the angle of incidence at which the wave vector and frequency of the evanescent field match those of surface plasmons at the metal-air interface. The most obvious manifestation of this coupling is a drop in the intensity of the reflected light.
In practice, surface plasmon sensors have some limitations. Because of its sharp resonance, silver would appear to be the best material for making sensors with high surface sensitivity. However, because silver is chemically reactive it is not suitable in most applications. Similar reactivity issues eliminate copper and aluminum. Gold is thus the standard material for essentially all commercially available surface plasmon sensors. However, gold has a less well-defined resonance than silver because of its higher dielectric loss. Gold films have limited sensitivity to dielectric changes at the surface because of the difficulty of accurately detecting small angle shifts of the broad resonance. Furthermore, the optical properties of gold mean that it only supports surface plasmons at longer wavelengths in the red and infrared.
Microarray-based assay methods also have become a mainstay of biological research, particularly in the areas of genomics and proteomics. Existing microarray readout technologies, however, have several limitations. For example, current microarray methods use fluorescent, colorimetric, or radioactive tags or labels to detect binding. Fluorescent labels currently are the most common detection strategy, but suffer from problems of low sensitivity, high background interference, and cross-reactivity. These problems are exacerbated in the case of proteins where the presence of a fluorophore can alter a protein's binding properties.
Accordingly, what is needed is an optical sensor apparatus that circumvents the limitations imposed by chemical reactivity, dielectric loss, and wavelength operating range. Further, a microarray-based sensor apparatus is needed that avoids the problems of low sensitivity, high background interference, and cross-reactivity associated with existing microarray technologies.